For the treatment of vascular lesions, particularly in cerebral vessels, with stents or generally tubular structures, it is expedient for the latter to have a high degree of flexibility. A high degree of flexibility improves the behavior of the stent in tightly curving vessels. When the stent curves, it lengthens, and the areas of the stent located on the outside of the vessel curvature expand or stretch more than the areas arranged closer to the center of the curvature. There is therefore a relationship between the flexibility, in the sense of being able to curve or bend, and the change in length or the maximum lengthening of the stent.
FIG. 1 is a schematic representation of the problem addressed by the invention. FIG. 1 shows a blood vessel comprising a first vessel portion 5a and a second vessel portion 5b, wherein a vessel curvature with a relatively small angle is formed between the first vessel portion 5a and the second vessel portion 5b. This means that the vessel curvature or vessel wall curvature between the second vessel portion 5b and the first vessel portion 5a is relatively tight. In order to place a stent in this area, it is therefore necessary that the stent is able to follow this vessel curvature. The required ability of the stent to curve or bend for this purpose is referred to in the context of the invention as flexibility.
Known stents comprise a lattice structure, which is produced from a tubular solid material by means of a method in which material is removed. As is shown in FIGS. 2a and 2b, the lattice structure 1′ comprises a multiplicity of cells, which are defined by webs 2′. Here, the webs 2′ are interconnected at an angle and form a rhomboid structure. In known stents, the degree of the change in length or the maximum lengthening is determined by changing the rhombus angle α, β. It is advantageous if the rhombus angle α in the rest state is relatively large (FIG. 2a) and the rhombus angle β in the stretched state is relatively small (FIG. 2b).
However, with a large angle difference between the rhombus angle α in the rest state and the rhombus angle β in the stretched state, there is a danger of the stent material undergoing plastic deformation at the connection points of the webs 2′. Moreover, a large rhombus angle α in the rest state of the stent means that considerable force has to be applied in order to bring the stent to the compressed state. This problem can be managed by reducing the volume of material at the connection points, for example by reducing the web width.
In stent production methods based on a laser cutting process, in which the lattice structure is formed from a tubular solid material by means of removal of material, the smallest possible dimensions of the web are limited on account of the thermal effects along the cutting edges.
In order to achieve smaller web dimensions, it is known to form the lattice structure of the stent by an etching method, with preference being given to the use of wet chemical etching processes, which permit a high speed of production. As is shown in FIG. 3, a web layer 2″, which comprises the material of the lattice structure that is to be produced, is applied to a substrate layer 3′. The web layer 2″ is applied in the layer thickness corresponding to the later web thickness. Moreover, a photoactive layer 4′ is applied to the web layer 2″ and, after suitable photo-lithographic treatment, forms an etching mask for the wet chemical etching process.
However, the wet chemical etching process also causes a lateral etching of the web layer 2′ or an undercutting of the photoactive layer 4′. This means that the web layer 2″ is also partially removed underneath the photoactive layer 4′. Consequently, the webs of the lattice structure that are produced by such methods have a trapezoidal profile, in which case relatively sharp edges form on the trapezoid base and can have a negative effect on the function of the stent. In particular, there is a danger of the sharp edges of the trapezoidal profile injuring the vessel walls or having a negative impact on the flow of blood in a blood vessel. Moreover, the webs produced by the wet chemical etching process according to the method known from the prior art have a relatively large web width, particularly on the trapezoid base, with the result that the flexibility of the known stent is further limited.
Therefore, in document WO 2008 000 467 A1 mentioned in the introduction, a method is proposed that permits the production of a lattice structure with increased edge precision of the webs. In this method, a sacrificial layer, which is structured using a photolithographic etching mask, is first of all applied to a substrate. The sacrificial layer is structured by a dry etching process in order to achieve the high degree of edge precision. In a further step, the substrate is subjected to a wet chemical etching process, as a result of which the sacrificial layer is undercut.
After removal of the etching mask, the stent material is then applied, in a sputtering process, to the laminate composed of substrate layer and sacrificial layer, with the stent material gathering in part on the sacrificial layer and in part on the etched substrate material. Following the removal of the substrate layer, of the sacrificial layer, and of the stent material embedded in the areas of the substrate layer etched by wet chemical etching, the desired self-supporting lattice structure remains.
A disadvantage of the method in WO 2008 000 467 A1 is that the sacrificial layer material, which preferably comprises gold, copper or chromium, is no longer available for the production process after removal or can only be reused by means of a complicated recycling process. Consequently, the known method is relatively expensive. Moreover, the removal of the sacrificial layer involves an additional method step, as a result of which more time is needed for the production of a stent.